1. Field of the Invention
The invention relates to radar systems and more particularly, to instantaneous automatic gain control systems, particularly applicable to coherent radar systems.
2. Description of the Prior Art
In the prior art, both coherent MTI (Moving Target Indicator) radar and IAGC (Instantaneous Automatic Gain Control) are known in a variety of forms.
The textbook entitled "Radar Handbook" by Merrill I. Skolnik, (McGraw Book Company 1970) provides a relatively current assessment of the state of these arts, along with extensive bibliographic references for more detailed study thereof.
The aforementioned Radar Handbook treats the subject of MTI Radar in Chapters 17 and 18. Moreover, Chapter 19 of this same text deals with Pulse-Doppler Radar, which may be thought of as another form or variation in the general art of moving target detecting radars.
In basic terms, the so-called coherent MTI radar systems as a class, to which the present invention particularly applies, may be thought of as relying on the Doppler phenomenon. Coherent MTI radar requires the use of stable transmitting and receiving components. In the usual pulse magnetron radar transmitter, the radio frequency starting phase cannot be duplicated from pulse-to-pulse, however, a so-called coherent oscillator (COHO) is used, usually in the IF domain, to "remember" the transmitted phase for later comparison purposes. Thus, a phase detector may be employed to reduce the received signals through the intermediate frequency amplifier to the video domain, the said phase detector operating against the aforementioned COHO to produce bi-polar video output pulses. The instantaneous video polarity and amplitude corresponding to the echo of a given moving target results, in effect, from modulation at the Doppler frequency corresponding to the velocity of the moving target. The bi-polar video amplitude thus discretely assumes an amplitude varying (from pulse repetition period interval to pulse repetition period) from a maximum positive amplitude to a maximum negative amplitude at the said Doppler rate. In a bi-polar video signal train containing a plurality of moving target echo signals, the instantaneous signal amplitudes within any one given pulse repetition interval are thus not the same, even if the reflected energy from them were substantially the same.
In the MTI canceller, the full repetition period delay device and the differencer provides for subtraction of one video train from that of the next pulse repetition interval. Accordingly, the quality of cancellation achieved is critically dependent upon the preservation of video waveforms and the avoidance of various forms of distortion.
One of the types of distortion which can result in severe MTI performance degradation is spectral spreading caused by overloading, of the IFF amplifier for example, in the receiver circuitry.
Various forms of gain control have been implemented in order to deal generally with this problem, but most of these are either operative within the IF amplifier itself, or not generally applicable to the coherent radar problem if they rely on a pulse feedback arrangement because of the aforementioned variation in echo pulse amplitudes from the system phase detector.
Since signal strengths follow the inverse fourth power law in transmit-receive radar echo ranging, compensatory variation of the receiver gain throughout each pulse repetition interval is effected by applying a gain-controlling signal which gradually varies throughout at least a part of each pulse repetition period. This is generally referred to as STC (Sensitivity Time Control). STC systems are also known and are described, for example, in the aforementioned Radar Handbook in Chapter 5, and Section 5.6.
The manner in which the present invention provides a unique, improved and useful pulse-to-pulse gain control in the receiver of a coherent MTI radar system, will be understood as this description proceeds.